1. Field of the Invention
The present invention relates to the field of digital electronic cameras and more specifically to a method for focusing by dynamic subsampling of the electronic image when using a reduced resolution electronic display.
2. The Prior Art
All cameras, except the simplest cameras that use small aperture or pin-hole lenses, require focusing of the lens in order to optimize the useable camera-to-subject range for a given lens aperture. As the lens aperture is increased to provide greater light gathering, the depth of field (range over which a subject is acceptably in focus) decreases causing the focus adjustment to be more critical.
A wide lens aperture is also used as a means for emphasizing selected portions of an image by limiting the depth of focus range to include only the selected portion. For this reason, the photographer needs to be able to sharply focus a selected portion of the subject scene so that the portion of interest is within the depth of field for the lens aperture in use.
The lens focusing is normally done while watching the image sharpness directly or through some kind of focusing aid using eye-to-hand feedback control. Conventional film cameras employ a variety of optical focusing aids such as magnifiers and split-image focusing prisms. Digital electronic cameras (hereinafter simply referred to as xe2x80x9cdigital camerasxe2x80x9d can also use the same type of optical viewing and focusing devices. Digital cameras that incorporate an electronic display with a resolution that equals (or exceeds) the full resolution of the electronic image sensor can be focused by directly viewing the image sharpness while adjusting the focus.
The present invention uses a focusing method for digital cameras with electronic viewfinders that have an electronic display, such as a liquid-crystal-display (LCD), with a resolution that is less than the resolution of the internal electronic image sensor array. Image sensor array resolution is usually expressed in terms of the number of xe2x80x9cpixelsxe2x80x9d used to represent the image or, equivalently, the number of photocells in the image sensor array. Because the number of pixels in the image sensor array is greater than the number of pixels that can be displayed, focusing is typically done only on a selected subset of pixels by using a zoom (or magnifier) mode. The zoom mode displays a full resolution image by selecting a subset of contiguous pixels with a pixel count equal to the electronic display capabilities.
The present invention provides a focusing aid and method for use on an electronic display with reduced resolution that allows the photographer to interactively focus on any region of the image while viewing the full image.
A dynamic image display method for high-resolution digital cameras, when viewing is done with a reduced-resolution electronic viewfinder display, includes the steps of:
(a) selecting a subset of pixels for producing a reduced resolution image with uniformly reduced horizontal and vertical scales;
(b) displaying the reduced resolution image pixels on a reduced resolution electronic viewfinder display for at least one display frame;
(c) generating a dynamic display by using a different subset of uniformly spaced pixels each time a new lower resolution image frame is displayed; and
(d) producing a periodic sequence of such reduced resolution images by cycling through a prescribed set of pixel subsets.
The dynamic focusing method of the present invention produces a periodic display of a set of reduced resolution images that exhibit the effects of spatial frequency aliasing, which provides a useful aid to focusing.
A digital electronic camera is also disclosed that has a live viewfinder to aid in focusing that includes a row and column address generator for subsampling the image sensor pixel array for display in a viewfinder with a lower resolution. The subsampling is accomplished at substantially uniform intervals by a programmable row and a column address generator so that scaling is substantially uniform in the horizontal and vertical directions of the image. The live viewfinder displays a dynamic periodic sequence of different subsampled images of the entire image as an aid to focusing.